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Astrobiology: Water and the Potential for Extraterrestrial Life

Astrobiology is a new interdisciplinary science that seeks to understand
the origin, evolution, distribution, and future of life in the universe.
As a fundamental requirement of living systems, water holds a special
place in the conceptual framework of astrobiology. All of life's
processes are carried out in the presence of liquid water, and on this
basis it may be regarded as a key indicator for potential habitability.
The importance of liquid water as an organizing principle in the
exploration for
extraterrestrial
life often is articulated in the simple expression "follow the
water."

Water and Planetary Habitability

What is it about water that justifies its central role in the search for
extraterrestrial life? Most of water's unique properties (e.g.,
its excellent
solvent
properties, broad temperature range over which it remains liquid, high
heat capacity, and surface tension) are rooted in the ability of water
molecules to form
hydrogen bonds
with each other. In addition, on freezing, there is a slight expansion
of hydrogen bond angles that produces a solid phase (ice) of lower
density than the liquid phase. This uncommon property results in
waterbodies that freeze from the top downward, an important factor for
sustaining habitability in polar and other cold climates.

Clearly, a knowledge of the past and present distribution of water in
the solar system is regarded as crucial for evaluating the potential of
other planets (or their moons) to develop and sustain life. Water also
holds central importance in the human exploration of the solar system,
being essential for the colonization of other planets, such as Mars.

Global Cycles.

Throughout Earth's history, water has played a central role in
the global cycles that link the solid Earth and the atmosphere.
Interactions between crustal rocks and water sustain a broad range of
processes that collectively meet most of the important energy and
resource requirements of living systems. Such interactions ultimately
determine the overall habitability of a planet, thus setting the stage
for life's origin and ensuring its persistence over geologic
timescales.

The hydrologic cycle (the cycling of water between the atmosphere and
oceans) drives a vast transport system that constantly redistributes
materials and energy within the Earth's crust. Flowing water and
ice transport rock fragments and associated
weathering
products from source areas to basins of deposition. Streams and
groundwater
(inclusive of hydrothermal systems) dissolve, transport, and
concentrate chemical compounds required by organisms.

Giant meteorite impacts billions of years ago would have caused
setbacks in early Earth's ability to develop a stable
biosphere. Yet once these impacts lessened, water and organic
materials needed for living systems could have been retained. Most
of the important materials for life on Earth (e.g., chemical
nutrients) could have been brought in by comets and other icy
objects.

Sediments
and the dissolved materials formed during weathering processes
ultimately reach the
ocean basins
, where they accumulate as dissolved salts or seafloor sediments. Over
the long term, even the dissolved load of streams eventually
precipitate
out of solution as secondary minerals (such as sedimentary cements) and
chemical sediments (such as
evaporites
). The deposits so formed often preserve signals for environmental
change on Earth along with a fossil record of life's evolution.

Over longer spans of time, cycling of the crust by the
subduction
of lithospheric plates and melting of sediment-covered seafloor and
entrapped sea water produce magmas (molten rock materials). The water
dissolved in these magmas actually lowers their density and
crystallization temperature, thus promoting their buoyant rise back to
the surface, where they drive volcanic activity.

Outgassing.

The Earth's close orbital distance from the Sun ensures a vast
supply of solar energy that is utilized by photosynthetically based
surface ecosystems. However, the energy output of the Sun was probably
much lower (30 percent less than present luminosity) at the beginning of
solar system history.

Under these relatively faint young-Sun conditions, an atmospheric
greenhouse, sustained by carbon dioxide (CO
2
) and/or methane (CH
4
), was required to maintain habitable surface conditions. An active
plate tectonic
cycle over the entire history of Earth has allowed for the constant
renewal of the atmosphere by volcanic outgassing. This atmospheric
renewal is essential for long-term sustainability. (By contrast, see the
discussion of Mars farther ahead in this entry and elsewhere in the
encyclopedia.)

By approximately 2.5 billion years ago, interactions between the global
hydrologic system and geologic cycles of the solid Earth (via processes
such as plate tectonics, weathering and erosion, and volcanism) had
produced a

Surface features of Mars suggest ancient activity of liquid water,
and groundwater may currently exist deep in the subsurface.
Scientists plan further explorations for evidence of past and
present water, and perhaps Martian life.

clear compositional differentiation of the Earth's habitable
surface environments into two broad habitats: the
continental
land masses and the ocean basins. Around the same time, oxygenic
photosynthesis
emerged as a major biological innovation, taking advantage of the
abundant energy available from the Sun. Oxygen production through
photosynthesis eventually outstripped volcanic and weathering controls
on atmospheric composition, producing an oxidizing surface environment.

By approximately 600 million years ago, the buildup of oxygen in the
atmosphere culminated in the appearance of large, multicellular life
forms. This new level of organization in the biosphere enhanced global
biodiversity
, leading in stepwise fashion to the emergence of terrestrial
(land-based) faunas and eventually to intelligent life characterized by
self-awareness and advanced cultural, social, and technological
civilizations.

Exploring for Martian Life

Given the terrestrial experience of humans, it is easy to understand why
the search for water in all its forms, past or present, has emerged as
the primary theme for exploration of the solar system. For example, over
the next decade, scientific efforts to explore for water on Mars will
create a context for assessing planetary habitability and the potential
for Mars having developed life at some time in its history.

Presently, the surface of Mars is properly regarded as a radiation-rich
frozen desert that is hostile to life. Within about 1 billion years of
its origin,
Mars appears to have lost most of its atmosphere and, with that, the
potential for sustaining liquid water environments at the surface.
Interestingly, this early loss of the atmosphere appears to have been
the result of the absence of a plate tectonic cycle on Mars.

Yet Mars has not always been a dry, hostile place. Exploration efforts
in the late twentieth century revealed that prior to the loss of its
atmosphere, Mars probably was much more Earth-like. The ancient southern
highlands of Mars harbor a wide variety of water-carved landforms and
layered sedimentary deposits of likely aqueous origin. The broad
temporal distribution of these features suggests that even though the
surface of Mars has been dry for most of the planet's history,
liquid water has been present from time to time, providing brief
intervals of surface habitability.

Loss of the Martian atmosphere would have spelled doom for any surface
life existing at the time. However, if Martian life forms colonized
surface environments during earlier wet periods, they are quite likely
to have left behind a
fossil
record. The search for this fossil record is in many ways the focus of
the current Mars exploration program.

Recent Discoveries.

The possibility of living Martian life-forms is one facet of ongoing
research. On Earth, scientists have discovered that life occupies an
incredible range of environmental extremes, including the deep
subsurface, where it utilizes chemical energy instead of sunlight.
Models suggest that liquid water (perhaps saline) environments could
still exist today in the deep subsurface of Mars, along with
energy-containing compounds such as methane, which could sustain
chemically based life. The argument for subsurface habitability is
strengthened by the existence of ancient out-flood channels, believed to
have been formed by catastrophic releases of subsurface water in the
past. These landforms provide direct evidence that a groundwater system
once existed.

But what about today? Scientists recently discovered what appear to be
water-carved gullies on the steep slopes and high latitudes of Mars.
Despite the constant subfreezing temperatures at those latitudes, water,
in the form of subsurface hydrothermal
brines
, may have risen from deep crustal sources along
faults
, flowing briefly over the surface and carving the channels.

The origin of these seep features remains controversial, but the
hydrologic interpretation is consistent with a variety of other types of
evidence that suggest the presence of a subsurface groundwater system.
Further investigation of these features is warranted. If a subsurface
groundwater system does exist on Mars, such environments may have
provided stable habitats for life over the entire history of the planet.
In 2002, the gamma-ray spectrometer onboard NASA's Odyssey
orbiter discovered extensive water present as ground ice in surface
soils over extensive regions of Mars at high latitudes. This has
strengthened the case for an abundance of subsurface crustal water on
Mars.

Research Challenges.

In exploring for Martian groundwater, the practical problem faced by
NASA (National Aeronautics and Space Administration) is accessibility.
Accessing and sampling sources of subsurface Martian water (and
potentially life) will require the development of precision landing
systems capable of safely landing on steep slopes where potential seep
sites are located, and/or long-ranging rovers capable of traveling to
prospective
groundwater sites (such as seeps) from safe landing sites located at a
distance of perhaps tens of kilometers. Next, scientists will need to
drill to depths of tens to hundreds of meters from small robotic
platforms, a capability they presently lack.

Although the previously mentioned technological capabilities have all
been identified as long-term goals of NASA's Mars exploration
program, scientists presently lack the technologies needed to access
subsurface water on Mars with robotic platforms. As a result, some have
suggested that drilling for Martian groundwater may require a human
presence, something that is beyond the scope of the present Mars
program. The earliest human missions to Mars, if they can be safely
carried out, are unlikely to occur prior to 2025.

LIFE IN A MARTIAN METEORITE?

About 20 percent of the magnetites found in a 4.6-million-year-old
Martian meteorite named ALH84001 resemble intracellular magnetites
formed by some species of terrestrial bacteria. (Magnetite is a
naturally magnetic mineral common in basalt.) Whether the meteoritic
magnetites are a reliable indicator of life was under scientific
scrutiny as of 2002.

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